The present invention relates to an epoxy resin composition. More particularly, the invention pertains to an epoxy resin composition comprising an alicyclic epoxy resin, a curing agent having phenolic hydroxyl group in its molecule and an imidazole curing accelerator.
It is well known that epoxy resins are widely used as electric insulating materials, for example, as an insulating material for conductors of rotary electric machines. There are provided various epoxy resin compositions, any one of which is far superior to the other thermosetting resins such as phenol resin in properties such as electrical properties and thermal resistance. However, the epoxy resin compositions are considerably inferior to silicone resins particularly in thermal resistance, and are still unsatisfactory as an insulating material for large rotary electric machines which require excellent thermal resistance, for example, thermal resistance of at least class H (at least 180.degree. C). Also, as a resin used in the prepreg insulation method which is recently often employed, the epoxy resin compositions are unsatisfactory in the storage stability of resin compositions (varnishes), curability, flexibility after semicure, that is, when formed into a prepreg tape, and thermal resistance (heat deterioration characteristic) and mechanical strengths, particularly bending strength, after cure. Particularly, there is no epoxy resin composition which can be used as a resin for prepreg insulation method having thermal resistance of at least class H.
An object of the present invention is to provide an epoxy resin composition which can give a cured product having excellent thermal resistance and excellent mechanical properties.
Another object of the invention is to provide an epoxy resin composition which shows good flexibility in a dry state or a semi-cured state.